1. Field of the Invention
The present invention relates to a hydrodynamic bearing system which is suitable to be used as a bearing for a rotary unit of a deflection scanning apparatus used with a recording system such as a laser beam printer.
2. Related Background Art
Recently, rotary apparatuses which must be operated at a high speed and/or with high accuracy have been requested more and more. Particularly, in a laser beam printer, a hydrodynamic bearing of non-contacting type has been used to obtain the rotation with high accuracy. Such a hydrodynamic bearing is described, for example, in the Japanese Patent Application No. 1-35564 filed on Feb. 15, 1989.
Such hydrodynamic bearing system is designed as shown in FIG. 15. That is to say, it comprises a shaft 1 having herring-bone shallow grooves formed in a radial (peripheral) surface thereof, a sleeve 2 rotatably receiving the shaft 1, and a thrust plate 3 fixed to the sleeve 2, for holding the shaft in the thrust direction; shallow spiral grooves 11 being formed in an end face of the shaft 1 and in an opposed surface of the thrust plate 3, and a space defined between the shaft 1, sleeve 2 and thrust plate 3 being filled with lubricating fluid. When the shaft 1 is rotated, the lubricating fluid flows in the direction shown by the arrows. Accordingly, in the radial direction, the pressure is generated by the herringbone shallow grooves 10 to maintain the radial surfaces of the shaft and sleeve in a non contacting condition. Also, in the thrust direction, the pressure is generated by the opposed shallow spiral grooves 11 to support the end of the shaft in a non-contacting fashion. Further, the thrust plate 3 has a central hole 7 and peripheral holes 9, these holes being communicated with each other by grooves or recesses 8. In this way, the lubricating fluid is circulated, thereby preventing the occurrence of negative pressure at the peripheral area of the bearing forming portion and permitting the dispersion of heat.
Further, an annular recess 5 is formed in an inner peripheral surface of the sleeve 2 in the vicinity of an open end thereof, and, at an outside of the recess 5 (i.e., at a position nearer to the open end of the sleeve than the position of the recess), shallow grooves 6 are formed in the radial surface of the shaft 1 into which the fluid can flow in the thrust direction, thereby preventing the scattering of the fluid.
Further, in order to stabilize the floating feature of the shaft, small holes 12 may be provided in the vicinity of the recess 5 (at a position corresponding to one between the shallow grooves 10 and the shallow grooves 6).
Further, FIG. 16 shows a sectional view of a deflection scanning rotary apparatus of a laser beam printer incorporating the hydrodynamic bearing therein. In FIG. 16, a rotary shaft 101 is rotatably received in a sleeve 102, and a thrust plate 103 and a fixing plate 104 are arranged at a lower end of the sleeve 102 and the fixing plate is fixed to an outer housing 105. The rotary shaft 101 has a flange 106 fixedly mounted thereon. A rotary polygonal mirror 107 is fixed to the upper side of the flange 106, and a yoke 109 having driving magnets 108 fixed thereto is fixedly mounted on the lower side of the flange. Further, stators 110 are fixed to the housing 105 in confronting relation to the driving magnets 108.
The thrust plate 103 has a shallow groove 111 on its surface opposed to the end of the rotary shaft 101 to form a thrust hydrodynamic bearing, and also has a central hole 112 and radial recesses 113 for the circulation of the lubricating fluid. In addition, on the outer peripheral surface of the rotary shaft 101, two sets of shallow herring-bone grooves 114 are provided in confronting relation to the inner peripheral surface of the sleeve 102 to form a radial hydrodynamic bearing. Further, in the vicinity of the open end of the sleeve, shallow spiral grooves 115 are formed in the outer peripheral surface of the rotary shaft to flow the lubricating fluid toward the thrust hydrodynamic bearing. Further, an annular recess 116 formed in the inner peripheral surface of the sleeve 102 in confronting relation to a position between the shallow herring-bone grooves 114 and the shallow spiral grooves 115 of the rotary shaft, the recess 116 having small radial holes 117. In this way, the stability of the hydrodynamic bearing portions is ensured.
However, in such system, if the thrust plate is made of metallic material, the machining operation is required, which machining operation meets with the working accuracy of the thrust plate, but is costly. On the other hand, if the thrust plate is made of resin material (such as polyacetal, polycarbonate, PPS and the like), the shrinkage of the resin after it is moulded must be taken in consideration. If not considered, the shape or configuration of the thrust bearing portion is undulatory or uneven, which results in the difficulty in the insurance of the accuracy of the thrust plate itself and/or the assembling of the thrust plate. Further, the contacting points between the rotary shaft and the thrust plate when the shaft is stationary is unstable, with the result that the frictional torque would be increased when the shaft starts to rotate. Furthermore, the eccentric or local where the torque would occur, and the ability for generating the thrust force when the shaft is rotated would be reduced, thus making the bearing efficiency unstable.
Further, if the above-mentioned hydrodynamic bearing is used with the laser beam printer, when the deviation of the inclination of the rotary polygonal mirror is increased, the quality of an image formed by the polygonal mirror will be worsened. In order to reduce such deviation, if the clearance or distance between the spiral grooves formed in the shaft is decreased, the loss of the torque will be increased when the shaft is rotated at a high speed. Further, if the clearance is merely decreased, the flow rate of the fluid or the generated pressure will be increased, thus worsening the stability of the thrust hydrodynamic bearing.